Ferritic stainless steel

ABSTRACT

A ferritic stainless steel containing 18 to 35% chromium and having a very low carbon and nitrogen content, the total of the two preferably being not greater than 0.01%, has a molybdenum content of from at least 0.5% and up to 6.0%. As compared to prior art ferritic high-chromium stainless steels, this new steel gives substantially greater resistance to corrosion by fluids containing chlorides, higher notched bar impact and tensile test values, has in general substantially improved cold formability, and has other advantageous properties which may be increased by the addition of alloying elements.

United States Patent 1 Brandis et a1.

FERRITIC STAINLESS STEEL Inventors: Helmut Brandis; Rudolf Oppenheim,

both of Krefeld; Gustav Lennartz,

'Sonnenause; Heinrich Kiesbeyer, Krefeld, all of Germany Assignees: Deutsche Edelstahlwerke GmbH,

Krefeld, Germany Filed: on. 26, 1972 Appl. No.: 301,067

Foreign Application Priority Data Oct. 26, 1971 Germany 2153186 Oct. 28, 1971 Germany 2153766 References Cited UNITED STATES PATENTS 6/1938 Espe 75/126 Dec. 24, 1974 2,141,016 12/1968 Payson 75/128 W 2,200,545 5/1940 Field 75/126 C 2,624,669 1/1953 Craftsm. 75/126 C 2,624,671 l/l953 Binder 75/126 R 3,562,781 2/1971 Tanczyrl.... 75/125 3,713,812 l/1973 Brickner... 75/126 C 3,726,668 4/1973 Baumel 75/126 C Primary ExaminerC. Lovell Assistant'Examiner-Arthur J. Steiner Attorney, Agent, or FirmKenyon & Kenyon Reilly Carr & Chapin ABSTRACT A ferritic stainless steel containing 18 to 35% chromium and having a very low carbon and nitrogen content, the total of the two preferably being not greater than 0.01%, has a molybdenum content of from at least 0.5% and up to 6.0%. As compared to prior art ferritic high-chromium stainless steels, this new steel gives substantially greater resistance to corrosion by fluids containing chlorides, higher notched bar impact and tensile test values, has in general substantially improved cold formability, and has other advantageous properties which may be increased by the addition'of alloying elements.

1 Claim, 4 Drawing Figures FERRITIC STAINLESS STEEL BACKGROUND OF THE INVENTION sures containing corrosive liquids under pressure, have been considered to be mandatory. Their advantages are well known. However, they do have disadvantages, one being that they are very susceptible to stress-corrosion cracking.

The ferritic stainless steels are fundamentally resistant to stress-corrosion cracking as can be shown by testing them in boiling 42% magnesium-chloride solutions, or in a calcium-chloride solution having a mercury-chloride addition. Such steels contain adequate chromium to develop passivity as required to make them qualify as stainless steels. To meet severe operating conditions, they contain up to from to chromium, an example being Type No. 446 of the American Iron and Steel Institute Standard Type Numbers, this steel containing from 23 to 27% chromium.

However, prior art ferritic stainless steels become brittle when cold-worked; they give low notched bar impact and tensile test values. This brittleness is exhibited not only at subnormal temperatures but also at temperatures up to 100C. and even somewhat higher. Also, when heated, as during welding, and quickly cooled, they may be brittle.

Another disadvantage suffered by ferritic stainless steels has been that their general resistance to corrosion when subjected to fluids containing chlorides has been insufficient to permit their use when such conditions are encountered. This lack of resistance to such corrosion has prevented the use of ferritic stainless steels in many applications, the austenitic stainless steels being used instead.

SUMMARY OF THE INVENTION The object of the present invention is to provide a ferritic highchromium stainless steel which is free from the disadvantages of prior art ferritic high-chromium stainless steels, to a degree permitting it to compete more successfully with the austenitic stainless steels in all fields.

According to the present invention, this object is attained by a steel containing from 18 to chromium and having very low carbon and nitrogen contents, the total of the two preferably being not greater than 0.01%, and having a molybdenum content of from at least 0.5% and up to 6.0%, the balance being iron, the

. new steel optionally including certain additional alloying elements.

The carbon and nitrogen content in conjunction withthe high chromium content is insufficient to result in the formation of chromium carbides or nitrides at the grain boundaries of the steel, this resulting in the steel having adequately high notched bar impact and tensile test values even if the steel is rendered course grained by being heated. The molybdenum provides for adequate corrosion resistance when the steel is subjected to corrosive fluids containing chlorides.

, DESCRIPTION OF THE DRAWINGS FIG. I graphically shows notched impact strengths at differing temperatures;

FIG. 2 graphically shows tensile stength characteristics;

FIG. 3 illustrates a bent specimen of a sheet of steel; and

FIG. 4 graphically shows the pitting tendency offour steels.

DESCRIPTION OF THE INVENTION The ferritic high-chromium stainless steels of this invention have compositions within the following ranges:

18 to 35% chromium 6 to 0.5% molybdenum 0 to 5% nickel 0 to 2% copper 0 to 3% silicon 0 to 1% manganese 0 to 0.5% apiece of titanium, zirconium,

niobium or tantalum, boron. aluminum 0 to 0.015% carbon 0 to 0.015% nitrogen Remainder iron and impurities occasioned by smelting conditions In the above and throughout this specification and the appended claims, all references to compositional percentages are by weight.

With increasing chromium content, ranging from l8 to 25%, there is an increase in the passivity obtained by these new steels. Chromium contents below 18% do not provide sufficient passivity to be considered acceptable when meeting severe corrosion conditions; more than 35% of chromium does not appear to provide further improvement in passivity.

The low carbon and nitrogen contents prevent the formation of excessive compounds at the grain boundaries with the attendant trouble this might cause. Although the maximum content of carbon and nitrogen are specified at 0.015%, each it is preferable that their total should not exceed 0.01%.

The novel molybdenum content specified for use in these new ferritic stainless steels provides a very substantial and decisive improvement in their resistance to corrosive pitting and passivity. It is surprisingly effective in rendering the new steels generally resistant to corrosion when subject to the attack of fluids containing chlorides, this contrasting with the lack of resistance to such attacks by prior art ferritic highchromium stainless steels. At the same time their fundamental resistance to stress-corrosion cracking is not deleteriously affected.

With the lower chromium contents, the higher molybdenum contents are used, and with the higher chromium contents, the lower molybdenum contents are used, all while keeping within the specified ranges for the chromium and molybdenum contents.

It can be seen that the new steels essentially consist of the chromium contents. the maintenance of carbon and nitrogen at the low levels specified, and the molyb denum contents specified. The balance, is, of course, mainly iron, the reference to impurities being with the understanding that such impurities. must be kept below values that would produce a harmful effect on the properties of the steel. The optional use of the other alloying elements is explained as follows:

The use of up to 5% nickel, preferably 1.5 to 4%, increases both the cold-toughness and the corrosion resistance of the new steels, particularly when they must operate under reducing conditions.

As to the copper specified, its use of up to 2%, preferably from 0.05 to 1.5%, and of silicon up to 3%, preferably from 0.5 to 2%, has the advantage of further improving the resistance to corrosion.

The further optional additions of 0.5%, preferably 0.01 to 0.5% of the elements titanium, zirconium, either niobium or tantalum, improve the cold-toughness and machinability of the new steels. The same advantages are obtained by the optional addition of up to 0.5%, preferably 0.001 to 0.01 of boron. These additions also improve the weldability and the resistance of the steel to intergranular corrosion in the transition zones of welds.

As indicated, it is permissible to have present up to 1% of manganese and up to 0.5% of aluminum.

Because of their high chromium contents, the ferritic stainless steels made in accordance with this invention have great resistance to corrosion under oxidizing conditions as might be expected. In addition, and surprisingly, it has been found that these steels, even under reducing conditions, have excellent resistance to corrosion, far superior to those of known austenitic chromium-nickel steels which also contain molybdenum. 1n the following Tables 1, 2 and 3, for a ferritic stainless steel of this invention, containing 28% chromium and 2% molybdenum, the remainder being essentially iron, a comparison is made as to resistance to corrosion in boiling formic acid, acetic acid, and mixtures of them (Table l), in boiling phosphoric acid (Table 2), and in oxalic acid (Table 3), in the latter case at different temperatures and acid concentrations, the steel of the invention being compared with known austenitic chromium-nickel and chromium-nickel-molybdenum steels of the composition given in each case.

Table 1 Resistance to corrosion in boiling formic acid, acetic acid. and their mixtures (test lasted 24 hours) Weight loss in g/m? h 60 72 CH COOH Steel Material 10% 20% +10% No. CH COOH HCOOH 28% Cr. 2% Mo, 0.04 0

Rest Fe X5C1'Nil89 1.4301 0.14 1.22 1.24 X5 CrNiMo l8 10 1.4401 0 0.91 0.50

Table 2 Resistance to corrosion in boiling phosphoric acid (Test lasted 24 hours) Weight loss in g/m. h

Steel Table :.C2m nwl Corrosion resistance in oxalic acid. (Test lasted 24 hours) Weight loss in g/m". h

n.b. not determined Sp boiling point From the above Tables 1, 2 and 3, can be seen the superiority of the ferritic stainless steels falling within the previously specified ranges, and containing 28% chromium 2% molybdenum, the remainder substantially all iron, as respects resistance to corrosion, in comparison with the known austenitic chromium-nickel and chromium-nickel-molybdenum steels, the tests being made in various corrosive mediums.

It was not to be expected that ferritic steels of a composition within the ranges specified herein would remain more passive at substantially lower potentials than do the corresponding chromium-nickel and chromium-nickel-molybdenum steels having almost the same chromium content. Reduction mediums. which because of their great negative redox potential lead to activation and thus to dissolving of the high-chromium austenitic steel X5 CrNiMo 25 25, are not capable of activating the new ferritic chromium-steel having about the same chromium content; these mediums are even capable of stabilizing the passive state. Thus for example, in 16% H at 100C., the transition from the passive to the active state, for a steel containing 28% chromium and 2% molybdenum, is at 250 mVE whereas this potential, for the high-chromium austenitic chromium-nickel-molybdenum steel X2 CrNiMoN containing 25% chromium, 25% nickel and 2% molybdenum, the remainder being iron, lies at 75 mVE Material 50% mm, H PO, 70% 11,,P0, No. P A I P A P A 23% Cr, 2% Mo, 0.01 0.01 0.13 0.11 0.50 0.51 Rest Fe X2 CrNiMo I8 10 1.4404 0.35 n.b. 0.88 n.b. 3.5 n.b. X5 CrNiMoTi 25 25 1.4577 0.01 0.02 0.01/1.2 1.9 2.1 n.b. P air-passive use A used after activating with zinc n.b. not determined Table 3 60 Numerous chemical syntheses and reactions take place under conditions where the hydrogen potential is Corrosion resistance in oxalic acid. (Test lasted 24 hours) determulauve' Under such .condluons the potennal of weigh, [055 in h the passive steel surface ad usts itself to the hydrogen X5 X5 otential. Whereas at 'Acid Test 28% c1. 2% Mo. CrNiMo l8 CrNiMoTi 25 p such hydrogen. potfmlal Comm Temp Fe 12 25 austenitic/chromium-mckel and chromium-nickeltration molybdenum steels become active in aggressive medi- 40 0 0 0 0 0] ums (low pH value), and thus are able to suffer greater 5% 60 ,1 1, attack, the ferritic chromium-steels according to this so 0.01 0.30 0.07 invention, remain passive.

As has already been stated at the outset, up to the present time the use of ferritic stainless steels for the water, in the tensile test using smooth and notched specimens (notch number 3.0). Noteworthy is the high tensile strength/notch tensile strength ratio of 1.7. which goes below the value 1 only at lC.

occurs.

FIG. 2 shows the strength characteristics of two production melts of the steel 28/2 CrMo containing 0.002%C and also 0.0025% N, after a heat-treatment for 30 minutes at 875% C, followed by quenching in In a striking way it has now been found that ferritic FIG. 3 shows the example of a 4 mm thick metal chromium-steels made in accordance with this invensheet of the 28/2 CrMo steel containing C N equal tion have excellent mechanical characteristics, and in to or less than 0.01%. which was welded to other simi particular good notch impact strength and notch tensile lar material by the electric plasma-arc welding process. strength, as is shown by the following Table 4, and and then bent at a sharp 180 angle lengthwise of and FIGS. 1 and 2 of the drawings. In Table 4 below, are transversely of the welded seam. displaying previously shown the notch impact strengths for three steels unknown bending-toughness behavior for chromiumwithin the inventions range of composition, at temperch ferriti ee s. atures from l00C. to room temperature. In Table 5 are given the compositions of four differ- Table 5 Steel C Si Mn Cr Mo S P N C+N 7, a 72 72 a 71 /l A 0.003 0.01 0.01 19.9 4.85 0.008 0.005 0.001 0.004 B 0.001 0.01 0.01 23.8 3.43 0.008 0.005 0.001 0.002 c 0.002 0.01 0.01 28.1 2.11 0.008 0.005 0.001 0.003 o 0.004 0.01 0.01 l9.6 2.00 0.006 0.005 0.001 0.005

Table 4 ent steels within the alloying range given herein their resistance to corrosive pitting, in the range from 20 to Notch impact strength in mkp/cm (DVM specimen) I 100C being Shown in FIG- 4 I I u According to the general opinion held up to the presmeht ent time, molybdenum is with respect to resistance to Temp. 1 Steel Type CrMo W 20/5 CrMo corros on pitting about three times as efficacious as 0.002% c, 0.001% c. 0.003% c, chromium; that 1s each 1% of molybdenum has in this 0002? N 002% N 090W? N respect the same effect as 3% of chromium. Accord G00 09/10/07 ingly steel A containing 20% chromium and 5% molyb- 75 2152/07/10 35.5/1.3/37.9 2.0/0.7/0.8 denum should have about the same and apparently cer- 2 1/13/21 4h/ 40/39-3 -g tainly no more resistance to corrosion pitting as steel B Z, '8 384x813 lg/fg'fg fi which contains 24% chromium and 3.5% molybdenum.

+ 20 32l9/37.6/38.9 40/ 4o 32.8/3 12/311; This hitherto accepted assumption ought in actuality to be confirmed by FIG. 4 where there are plotted for the In this Table 4, which for each temperature for the four 3 A, B, C and D, their liability to pitting in 3% measurement of each type of steel givesthree mea- Nacl m a temperathrefangle f 10000 sured values of notch impact strengths, it is seen that 40 Cause of the almost slmllar P potenllalst steels At all specimens at room temperature are sure to have and should Show eqhahy h f P notch impact strengths of better than 30 mkp/cm? h Whereas D, because Of t poorer pitting po- Therefore it is possible to employ the steels of the intehhal accordlttgfo FIG Should let expect homer vention in a range of temperatures from room temperareslstahce to ture to higher temperatures, in cases where ability to 5 It s how thhhhd 9? a Shrphslhg h that h withstand corrosion, as well as good mechanical charaforesald h h h Incorrect h that m acthahty acteristics, and in particular notch impact strength, are the fohohimg hhdmgs were made: wlth h fohmahon of important encrustations, forming confined spaces m which corro- For the steel of Type 28/2 CrMo, there are plotted in h hlhds h trapphd that reglohs f hlgh FIG. 1 the notch impact strengths for temperatures g gh g fi g h H g ranging from c to 50C, and in FIG. 2 are plotaye e h 9 ted other mechanical characteristics in a temperature Both m boning-Seawater com-ammg from o O 2% to 10% NaCl, as half-immersed specimens, even range from l00 C to .+400 F these graph? h 15 after only a few hours, showed below the encrustations once more seen that m hP m hlgher 55 forming in a steam space substantial patches of corrotehhperhtures about, 400 femhc chrohhum Steels sion. Steel B behaved somewhat better showing the first of the present h h have exceheht Strength and corrosion phenomena only after some days. Steel A on toughness Chamctehshcs' a the other hand, in spite of being considerably en- I hh for low temphrathrhs of below T crusted, even after 2,000 hours showed no signs of a the trhhhlhoh temperathreh of the notch h h corrosive attack. Thus it has the best hehavior as re strength for l7 melts of th1s 28/2 CrMo steel containing Spec pining in ch|oride comaining mediums an 0.004 to 0.006% carbon and 0.00] to 0.004 nitrogen, expected result after a heat-treatment f0! minutes at 850 to 875C The proved excellent corrosion chemical characterfollowed by quenching in Water- In the region 0f room istics, and the described excellent mechanical-technotemperawre down to at least Cold-brittleness 5 logical characteristics, in particular the cold-toughness,

of the ferritic stainless steels of the range of composition to be used in accordance with this invention, form a secure basis for allowing the use of these steels for the building of pressure-tanks under specified obligations or warranties, and for making possible their use in a wide field of application in the chemical industry, and also in a general way in processes under reducing conditions, and in particular in the field of producing and processing organic chemical substances. The ferritic stainless steel of this invention is moreover suitable for ship building, for building appratus and equipment, such as heat exchangers, e.g., handling sea water, for pumps, piping, and the like.

What is claimed is:

l. A ferritic high-chromium stainless steel operating while stressed at temperatures ranging between lOC and 100C and higher while in contact with fluids containing chlorides and consisting by weight of:

chromium molybdenum boron nickel copper silicon manganese carbon nitrogen smelting conditions, 

1. A FERRITIC HIGH-CHROMINIM STAINLESS STEEL OPERATING WHILE STRESSED TEMPERATURE RANGING BETWEEN -100*C AND 100*C AND HIGHER WHILE IN CONTACT WITH FLUIDS CONTAINING CHLORIDES AND CONSISTING BY WEIGHT OF: 